The frontbenches are where ministers and their official Opposition spokespersons sit in the Chamber - this is where the term frontbencher or frontbench spokesperson comes from. It reflects the seating arrangements in both Houses as the leaders of the Government and the Opposition sit on the frontbenches, with their supporters, or backbenchers, sitting behind them.
Further information on this subject can be found from the following links.
Government and Opposition lists
Related glossary term: backbenches (backbencher)